Seattle's 1980 employment population of 124,000 is expected to jump to 150,000 by 1990. This will mean more commuter traffic and more mass transit coming into the city's already congested downtown. To accommodate this traffic, the city is planning to build a tunnel underneath the downtown business district to accommodate a system of special, dual-powered diesel-electric buses. The project will consist of twin tubes, with one 12 ft lane in each direction. Each bore will be approximately 20 ft in outside diameter. The tunnel will include five transit stations. In addition, street and sidewalk improvements will be made to improve surface transportation.